This invention relates to improvements in elastomer-based friction materials and more particularly to such friction elements, i.e., clutch, brake and the like, which run in oil and are referred to in the art as wet friction materials. In a typical application a specially prepared or compounded friction element, in the form of a disc, ring or band, is secured to a rigid backing plate, and the supported element is brought into engagement with a relatively moving reaction member to achieve power transfer or braking action.
Prior art wet friction elements have traditionally been composed of materials such as resin-impregnated paper, sintered bronze, or from a bonded mixture held together by a resin binder. Although such materials have proved to be highly useful over a wide variety of applications, the advent of larger and more powerful industrial vehicles has created for materials having improved energy absorption rates, better wear resistance, and a long service life at high operating temperatures.
More recently, certain elastomers have been used as a primary component and binder in wet friction materials, for example, as described in U.S. Pat. Nos. 3,898,361, 4,042,085, 4,045,402 and 4,131,590. Elastomer-based materials offer several potential advantages, inasmuch as the material offers greater elasticity than traditional materials and therefore exhibits better energy absorption rates. The relatively low modulus of the material may also allow the material to conform better to irregular or uneven surfaces of the reaction member without undue wear.
Of the conventional elastomers currently available, those offering high temperature stability and good oil resistance are generally preferred, such as cured nitriles and fluoroelastomers. As described in the foregoing patents, the elastomers may be compounded with organic and inorganic fibers for reinforcement, as well as relatively abrasive and non-abrasive fillers or socalled friction modifiers, and the elastomer is then cured to form a substantially uniform matrix with the fibers and fillers in the shape of a disc or pad.
In the testing of elastomer-based friction materials, it is commonly assumed that, for a given level of force, the static coefficient of friction will typically be substantially greater than the dynamic coefficient of friction, wherein the parts are still sliding against one another. In practical terms, if there is a large disparity between the static and dynamic coefficients of friction, the parts will tend to sieze up abruptly shortly before complete engagement. This in turn may cause the engagement to be unacceptably jerky and may cause chatter and noise.
In some applications, a smooth engagement throughout various levels of energy would be desirable, but heretofore in the case of fluoroelastomer-based materials, the static coefficient has been considerably higher than the dynamic, regardless of how the material was compounded and regardless of which materials, i.e., fibers and fillers, were used in combination with the elastomer.